1. Field of the Invention
The present invention relates to an image forming apparatus including a fixing unit which fixes a heated and melted unfixed toner onto a sheet of paper carrying a toner image while passing the sheet of paper between a nip by a heated roller pair or a heating belt and a roller.
2. Description of the Related Art
In this type of image forming apparatus, in order to meet demands such as shortening the warm-up time of a fixing unit and saving energy, attention has recently been drawn to a belt method capable of operating with a smaller amount of heat capacity (e.g., Japanese Patent Laid-Open Publication No. 6-318001). In recent years, an electromagnetic induction heating method (IH) capable of rapid heating or efficient heating has also been notable, and taking into account saving energy when fixing a color image, a large number of products created by combining the electro-magnetic induction heating and belt methods have been put on the market. The combination of the belt method and the electro-magnetic induction heating has advantages in that a coil can be easily laid out and cooled as well as a belt directly heated. These and other advantages frequently prompt an electro-magnetic inductor to be arranged outside of a belt (so-called external IH).
In the electromagnetic induction heating method, various arts have been developed for the purpose of preventing an excessive temperature rise in a non-sheet-conveyed region in accordance with the width (conveyed-sheet width) of a sheet of paper conveyed through a fixing unit. Particularly, a means for switching the size of a sheet of paper in the external IH is described in the following prior arts (e.g., Japanese Patent Laid-Open Publication No. 2006-120523). In the prior art, a magnetic-flux shielding plate having a curved-surface shape is formed in advance with a plurality of steps in the longitudinal directions thereof, and these steps form an area for passing magnetism and an area screening out magnetism in the width directions of a sheet of paper. Therefore, when the size of a sheet of paper is changed, the magnetic-flux shielding plate is rotated in accordance with the conveyed-sheet width, thereby screening out magnetism in a non-sheet-conveyed region to prevent a heated roller or the like from raising the temperature therein excessively.
However, in the prior art (Japanese Patent Laid-Open Publication No. 2006-120523), the positions of the steps formed beforehand in the shielding plate determine the shielding area and the non-shielding area, thereby making it difficult to handle sheets of paper having diverse sizes. Specifically, the prior art is capable of relatively easily handling one or two kinds of sheets having small sizes, but incapable of handling three or more kinds of sheets having small widths without devising the size of the magnetic-flux shielding plate or control of the rotation angle thereof.
In addition, if the steps are formed in the directions where the shielding plate rotates in the prior art (Japanese Patent Laid-Open Publication No. 2006-120523), then the rotation angle as a whole is restricted to hinder enlarging each step (e.g., a rotation angle of approximately 15°-30°), thereby reducing the quantity of screened-out magnetism and making it impossible to suppress the generated-heat quantity sufficiently.